Thermal steam scrubber



March 1968 E. A. CATLIN ET AL 3,373,544

THERMAL STEAM SCRUBBER Filed Jan. 15, 1964 2 Sheets-Sheet 1 INVENTORS Ambrose Cor: Jr.

ATTORNEY 2 Sheets-Sheet 2 FIG.3

FIG.4

TO FEEDWATER HEATER E. A. CATLIN ET AL THERMAL STEAM SGRUBBER TO AUXILIARIES March 19, 1968 Filed Jan. 13, 1964 FROM SUPER- HEATER United States Patent itice Patented Mar. 19, 1968 3,373,544 THERMAL STEAM SCRUBBER Everett A. Catlin, Morristown, N.J., and Ambrose Cort,

.Fra, Rye, N.Y., assignors to The Babcoclr d: Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Jan. 13, 1964, Ser. No. 337,239 6 Claims. (Ci. 55-269) This invention relates in general to the construction of apparatus for refining and drying gaseous fluids, and more particularly to separating and drying apparatus for the removal of entrained moisture and solids from the steam leaving the steam and water drum of a vapor generating unit.

The circulated water in a steam generating unit normally contains suspended solids introduced for the purpose of preventing scale formation and/or internal corrosion of pressure parts. A portion of the solids, either in vaporous or solid state, become contaminants within wet, saturated steam as it is generated. If entrained moisture and solids are allowed to pass with the steam from the steam and water drum to the superheater, as the temperature of the steam is increased, the impurities will disassociate from the steam either in the form of a vapor or as mechanically entrained minute solid particles. These impurities, carried by the vapor, are usually deposited on the internal surface of the superheater tubes as the vapor superheat is being increased, or on the working surfaces of an expansion type engine (such as a turbine) wherein the steam is being cooled. In either case, the deposition of these solids is extremely undesirable as it impairs heat transfer characteristics of the superheater and introduces tube failure hazards, or reduces the efiiciency and capacity of the expansion engine. In addition to the impairment of the efficiency, the deposition of these impurities ultimately results in increased outages and maintenance costs.

The functional nature of a steam and Water drum is such that two fluid phases coexist at the saturation temperature corresponding to the pressure in the drum. The task of separating the entrained liquid droplets from the vapor has generally been approached as a problem of mechanical separation. Various devices, such as cyclone separators, flow-directing baflie arrangements and combinations of these devices thereof have been used whereby the vapor is subjected to changes in direction of flow and/ or centrifugal forces to separate the relatively heavy moisture droplets from the vapor. These devices are normally located in the steam space of the steam and water drum,

and since the maximum temperature of the vapor is the same as the liquid saturation temperature, corresponding to drum pressure even nominal heat losses due to conduction through the drum shell will cause condensation of some of the steam. Thus, the presence of mechanical separation devices in the steam space of the drum tends to augment the amount of heat dissipated by conduction through the drum shell, and so may actually result in increased moisture and solids entrainment in the steam.

It is an object of the present invention to provide an apparatus whereby the steam leaving the steam and water drum is completely dried and free from entrained solids. The invention herein described is intended primarily for use in steam generating units for marine service; however, it should be understood that the inventive thermal steam scrubber is not limited to such use but may also be adapted to a wide variety of steam generator types.

In the present invention the steam is dried and the solids are removed in a combination scrubber and heating device, or thermal steam scrubber, which simultaneously heats the saturated vapor and subjects it to mechanical separation forces. While the scrubber could be located in a separate vessel external of the steam and water dru-m, it is preferably disposed in the steam space of the drum. The thermal scrubber herein described comprises a bank of spaced heating elements positioned in the steam space parallel to the longitudinal axis of the drum. A plurality of closely spaced, thin, corrugated scrubber plates are secured to the heating elements in heat conducting relationship along substantially the entire length of the elements. Bafiie plates are provided so that the vapor passing from the drum liquid level to the vapor outlet of the drum flows upwardly through the narrow spaces between the scrubber plates. The thermal effect of the scrubber is provided by heating the heating elements to a temperature substantially higher than the temperature of the steam being purifled. Although the heating elements may be of any suitable type, e.g. electric resistance heaters, in the preferred embodiment the thermal effect is provided by a high temperature fluid, such as superheated steam, supplied from an external source to flow through the tubes. The extended surface corrugated scrubber plates serve to effect an increase in the temperature of the saturated vapor passing therebetween, while at the same time evaporating the entrained moisture in the vapor. Simultaneously, the scrubber plates subject the vapor to mechanical separation forces, so that a portion of the entrained moisture is collected on the scrubber plates where it is evaporated, leaving the solids residue on the scrubber plates. As a result, the vapor passing from the scrubber plates to the vapor outlet of the drum will be slightly superheated and therefore dried, and will be free of solid impurities.

The corrugated scrubber plates may also be arranged so that the vapor flow passages between adjacent plates are of alternately converging and diverging cross-sectional flow areas, thus subjecting the flowing vapor to velocity and pressure changes to enhance separation action. By arranging the scrubber plates so that the tubes are located at the point of minimum flow area between adjacent plates, maximum heat transfer from the high tempera ture fluid to the vapor will be effected since the maximum mass flow of the vapor will occur at the point of highest temperature.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawing and descriptive matter in which there is illustrated and described a preferred embodiment of the invention.

In the drawings:

FIG. 1 is a cross-sectional view through a steam and water drum equipped with a thermal steam scrubber of the type herein disclosed;

FIG. 2 is a plan view of a thermal steam scrubber section;

FIG. 3 is a fragmentary side view of the thermal scrubber looking in the direction indicated by line 3-3 of FIG. 2;

FIG. 4 is an alternate arrangement of the structure shown in FIG. 3; and

FIG. 5 is a schematic diagram of a portion of the steam power system of a marine power plant incorporating the invention.

The invention will hereinafter be described with references to steam and water as the working substances, although it is not intended to be restricted thereto, and the words steam and water as hereinafter used are intended to refer to the vapor and liquid phases of working fluids generally.

In the drawing, FIG. 1, a thermal steam scrubber is shown disposed in the steam and water drum 12 of a steam generating unit (not shown). The drum 12 is formed as an elongated cylinder, being closed at its ends by appropriate drum heads (not shown). During normal operation, a liquid level 14 is maintained at about the horizontal centerline of the drum, the level 14 forming the boundary between the upper steam or vapor space 15 and the lower water or liquid space 16. Feedwater is introduced into the water space 16 through openings 17A in the feed pipe 17 which is submerged in the water space 16 and extends parallel to the longitudinal axis of the drum 12. The feed pipe 17 is held rigidly in place by U-bolts 18 which connect to the feed pipe support plate 19 that is welded to the drum 12. Water from the drum 12 circulates downwardly to the heat absorption areas of the steam generating unit through downcomer tubes 21. After the water is heated, the resulting steam-water mixture is discharged into the drum 10 through riser tubes 22.

To effect primary or first-stage separation of the steam from the water of the steam-water mixture entering the drum 12 through the riser tubes 22, and to maintain a stable water level 14, a submerged baflie 30, extending longitudinally of the drum 12 beneath the liquid level 14, is used to reduce the effect of the discharge velocity of the incoming steam-water streams. The baffle consists of three sections 31A, 31B and 31C, each of which extends throughout the length of the drum 12. The sections 31A, 31B and 31C are connected in side-by-side relationship so that all of the steam introduced into the bottom of the drum must pass through perforation in the baflie 30 in passing to the steam space 15. The outer sections 31A and 31C are canted upwardly toward their outermost edges and are connected to longitudinally extending support bars 32. The innermost edges of the sections 31A and 31C and both edges of the section 31B are connected to a pair of longitudinally extending support angles 33 which are supported on the transverse baffle support bars 34. Each bafiie section 31 consists of three perforated plates disposed in spaced parallel relationship and connected at their edges. 'The submerged baffle 30 tends to distribute the incoming steam throughout the water space of the drum to effect separation of the steam, while minimizing the tendency for violent agitation of the liquid level 14 and consequent moisture carryover. A submerged baflle 30 of this type, under normal operating conditions, has been found to be capable of limiting the moisture entrained in the steam leaving the drum 12 to a maximum of about .25 moisture by weight. Thus, the submerged baflie 30 is considered to be an excellent first-stage steam-water separator, effectively producing the requisite degree of primary separation.

It should be recognized that other types of first-stage or primary separators could be used if desired or required by the particular type of service. For example, in the vapor generators of naval vessels, where sudden and severe load changes and extreme operating conditions may be imposed, cyclone separators, as disclosed for example in U.S. Patent 2,732,028, may be used in lieu of the submerged b'affle 30.

Where desired, the final traces of moisture remaining in the steam may be completely eliminated by subsequently passing the steam through a second-stage purifier such as the thermal steam scrubber [0 as shown in FIGS.

4 1 and 2. It should be noted that this scrubbed 10 is to be used as a secondary steam purifying device in conjunction with a primary separator, such as the submerged baffle 30 or cyclone separators described above.

FIG. 1 shows a longitudinally extending thermal steam scrubber 10 centrally disposed in the steam space 15 of the drum 12. The scrubber 10 consists of a pair of heater sections 11, one of which is shown in FIG. 2. Each section 11 includes a row of co-planar, spaced tubes 40, the inlet ends of which are connected to and open into a supply header 41, and the outlet ends of which are similarly connected into an outlet header 42. The supply and outlet headers 41 and 42 are respectively fitted with centrally disposed inlet and outlet flanged connections 43 and 44 which connect to the high temperature heating fluid supply and discharge piping (not shown). The high temperature heating fluid passed through the tubes 40 may be any suitable fluid having a temperature higher than the saturat'oi temperature of the steam in the steam space 15 of the drum 12; however, the preferred high temperature fluid is superheated steam. A plurality of thin. closely spaced, parallel, corrugated scrubber plates 45 (see PG. 3) are attached to the tubes 49 along substantially their entire length, arranged perpendicularly to the longitudinal axis of the tubes 40. Each plate 45 is formed with horizontal corrugations and with suitably spaced openings through which the tubes 49 of one heater section 11 are received. The scrubber plates 45 may be attached to the tubes 4%) in any suitable fashion; however, it should be emphasized that the connection must be sufficient to insure good heat transfer from the tubes 40 to the plates 45. Preferably, the plates 45 are welded to the tubes 40.

The preferred arrangement of scrubber plates 45 is shown on FIG. 3. In this arrangement, the plates 45 are arranged in parallel relationship, and are so spaced that all of the steam passing therebetween will be subjected to undulating changes in the direction of steam flow and the consequent inertial and gravitational separating forces resulting from the difference in density of the vapor and the entrained moisture and solids.

A variant of the FIG. 3 arrangement of scrubber plates is shown in FIG. 4 where the plates 45A are arranged so that alternate pairs form flow passages of alternately converging and diverging cross-sectional area. In this arrangement the steam flows between only alternate pairs of plates 45A. It should be noted that the plates 45A are arranged for connection to the tubes 40 so that the smallest cross-sectional flow area (and highest velocity) is immediately adjacent the tube 40. By this device heat transfer to the steam is optimized since the highest mass flow rate occurs at the point of highest temperature.

The two sections 11 of the scrubber 10 (see FIG. 1) are connected at their adjacent inner extremities by a pair of longitudinally extending support bars 50 and longitudinally spaced bolts 51. The scrubber 10 is supported on the side baffle angle support plates 55 which extend the entire length of the scrubber 10 and are disposed adjacent and normal to the lateral extremities of the scrubber 10. The scrubber 10 is connected to the lower portion of the side baffle plates 55 by lugs 56 of the scrubber assembly and secured by bolts 57. It should be noted that the side baffle plates 55 form a vapor-tight barrier which prevents the steam from by-passin g around the sides of the scrubber 10. End baffle plates 60, positioned and secured between the top of the inlet and outlet headers 41 and 42 and the inner surface of the drum 12, adjacent the scrubber 10 are provided to prevent by-passing of steam around the ends of the scrubber 10. The side baffle plates 55 and end baffle plates 60, when connected, as by welding at their adjacent edges (at the corner of the scrubber 10), thus form a barrier which effectively directs the flow of steam upwardly through the corrugations of the scrubber 10.

In operation, a high temperature fluid, such as superheated steam at a higher temperature than the steam being generated, is introduced into the supply headers 41 and passes through the tubes 40 to the outlet headers 42. Since the scrubber plates 45 are connected to the tubes 40 in heat conducting relationship, the heat from the high temperature fluid is conducted to the plates 45 so that the plates 45 act as extended heat exchange surface. As the steam flows upwardly through the spaces between the plates 45 it contacts the plates 45 which are at a higher temperature than the steam being generated. At the same time, the steam is subjected to changes of direction due to the corrugated surface of the plates 45, whereby the moisture in the steam is subjected to separating forces tending to deposit the residual moisture and entrained solids on the plates 45. Since the plates 45 are heated by the high temperature fluid superheated steam flowing through the tubes 40, as the moisture collects on the plates 45, it is evaporated, leaving entrained contaminants, or solids, deposited on the plates 45. Additionally, the steam is heated by convection as it passes upwardly between the plates 45, so that the steam passing out of the drum 12 through the outlet 23 is dry, free of entrained solids, and preferably at a temperature slightly above the saturation temperature corresponding to the drum pressure, e. g., about 3 F. of superheat.

It should be recognized that alternate arrangements, employing the above disclosed thermal steam scrubber 10, may be used without departing from the spirit of the invention. For example a two stage scrubber, with the scrubber elements mounted one above the other, wherein the rising vapor passes serially through a pair of scrubbers while the heating fluid flows either serially through or in opposite directions through the two elements. These and other obvious minor changes to the disclosed arrangement are considered to be inherent in the present invention.

A pair of longitudinally extending spray pipes 70 are provided above the scrubber or heater sections 11 for intermittent back-washing of the deposits from the plates 45 and the exposed portion of tubes 40. The spray pipes 70 are held in place by U-bolts 71, suitably fastened to the support angles 72 which are secured to the drum 12. The spray pipes 70 are connected to a water source (not shown) and are provided with nozzles arranged to direct the flow issuing therefrom toward the scrubber plates 45 during the back-washing operation. Preferably this backwashing should be done When the steam generator is operating at low load or banked conditions to minimize moisture carry-over. Also, the flow of high temperature fluid should be temporarily terminated during backwashing.

To supplement the periodic back-washing of the scrubber 10, it may be mechanically cleaned during scheduled outages of the steam generating unit, at which time the access to the drum internals may be had through appropriate man-holes (not shown) in the drum heads.

Although the thermal steam scrubber has been described above as being located in the vapor space of the drum 17, it should be recognized that it could be located externally of the drum 12 in a separate pressure vessel (not shown). In such an installation, the wet, saturated steam would pass from the outlet 23 of the drum 12 to, the scrubber. Such an arrangement would be advantageous from the standpoint of affording relatively easy access for periodic cleaning of the scrubber, without actually removing the steam generating unit from service. However, it would require additional space for installation, and space, of course, is at a definite premium in marine power plant installations.

In marine installations, provisions are usually made for desuperheating a portion of the superheated steam output for use in auxiliary equipment, such as winch drives, etc. Although low pressure steam may normally be obtained more economically by extraction from the steam turbine, this is not practical in marine systems since the turbine runs only when the vessel is under way, and auxiliary steam is needed when the vessel is in port. To provide the necessary steam for auxiliaries, a desuperheater 80 (see FIG. 1) is provided. The desuperheater 80 consists of a bundle of tubes submerged in the water space 16 of the drum 12. Since the water in the drum 12 is always at saturation temperature, the desired steam tem perature for auxiliary steam can be obtained by simply controlling the steam flow rate through the desuperheater S0.

The schematic arrangement of FIG. 5 shows how the above described thermal steam scrubber 10 may be incorporated into the overall arrangement of a marine steam power plant. During normal operation, the major portion of the superheated steam flows from the superheater out let (not shown) through the main steam line 91 to the main drive turbine 90. A portion of the superheated steam is extracted from the steam main 91 and flows through the branch pipe 92. Some of this extracted steam flows to the thermal steam scrubber 10 via the line 93, while the remainder flows via line 94 into the desuperheater 80, these portions being in parallel flow relationship. The scrubber outlet line 95 connects into the desuperheater outlet line 96 so that the primary steam from the scrubber 10, while its temperature has been decreased, may be combined with the desuperheated steam for subsequent use in auxiliaries. A branch pipe 97 is also provided from the scrubber outlet line 95 so that the scrubber steam may be utilized for some other purpose, as for example heating feedwater. A desuperheater flow control valve 89 is provided in branch pipe 92 to control the total quantity of steam passing through the desuperheater. Control valves 98 and 98 are also provided in the lines 93 and 94 respectively to apportion the flow of superheated steam to the thermal scrubber 10 and the desuperheater 80. Shutoff valves 99 and 99 are suitably provided in lines 97 and 95 respectively so that the steam flowing from the scrubber 10 may be diverted from discharging into the desuperheater outlet line 96 and into line 97 for the alternate point of use.

From the above description it can be seen that, in marine installations, the thermal steam scrubber 10 can be employed in the overall power system without any appreciable loss in cycle thermal efficiency. Of course, depending on the particular installation, other piping arrangements may be more advantageous; however, in all instances where a portion of the main steam is to be desuperheated, the thermal scrubber 10 may advantageously be used without loss of thermal efficiency.

By way of example, and not limitation, if a thermal steam scrubber of the type disclosed is used in conjunction with a steam generator and superheater having a rated output capacity of 100,000 pounds per hour (at 900 F. and 875 p.s.i.g.), and if about 5,500 pounds per hour of this superheated steam is used in the scrubber to evaporate the moisture in the vapor and to heat the vapor to about 8.5 F. above the saturation temperature corresponding to the pressure in the drum 12, the fluid leaving the scrubber 10 will be at approximately 570 F. and 825 p.s.i.g. Of course, depending on the eventual use to which the desuperheated steam is to be put, the flow through the scrubber 10 can be regulated so that the desired outlet conditions are obtained.

While in accordance with the provisions of the statutes there is illustrated and described herein a specific embodiment of the invention, those skilled in the art will understand that changes may be made in the form of the invention covered by the claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.

What is claimed is:

1. In a steam generator, a steam and water drum of the type wherein a body of liquid having a water level is normally maintained, said water level defining the boundary between a lower water space and an upper steam space, means for discharging moisture laden steam into said body of liquid, means forming a steam outlet communicating with said steam space, a bank of spaced heating element-s disposed in said steam space between said water level and said steam outlet, a plurality of closely spaced thin plate members attached to said heating elements in heat conducting relationship along substantially the entire length thereof, said plate members being arranged relative to each other to form undulating flow channels therebetween, means for heating said plate members to a temperature substantially higher than the temperature of the steam in said stea-m space, and baflle means for directing the floW of said steam upwardly through said flow channels between said plate members, whereby separation of moisture from the steam passing through said flow channels is effected by virtue of directional changes of the steam flow, and the moisture thus deposited on said plate members is evaporated.

2. In a steam generator, an elongated cylindrical steam and water drum of the type wherein a body of liquid having a water level is maintained, said water level defining the boundary between a lower water space and an upper steam space, means for discharging moisture laden steam into said body of liquid, means forming a steam outlet communicating with said steam space, a bank of spaced tubular members disposed in said steam space between said water level and said steam outlet, a plurality of closely spaced corrugated thin plate members attached to said tubular members in heat conducting relationship along substantially the entire length thereof, said plate members being arranged relative to each other to form undulating flow channels of substantially constant crosssectional flow area therebetween, a source of heating fluid, said heating fluid being at a temperature substantially higher than the temperature of the steam in said steam space, means for passing said heating fluid from said source through said tubular members, and baflie means for directing the flow of said steam upwardly through said flow channels, whereby said steam is heated slightly above its saturation temperature and contaminantbearing moisture particles entrained in said steam are collected on said plate members, said moisture being evaporate-d while said contaminants remain deposited on said plate members.

3. In a steam generator, an elongated cylindrical steam and water drum of the type wherein a body of liquid having a water level is maintained, said water level defining the boundary between a lower water space and an upper steam space, means for discharging moisture laden steam into said body of liquid, means forming a steam outlet communicating with said steam space, a bank of spaced tubular members disposed in said steam space between said water level and said steam outlet, a plurality of closely spaced corrugated thin plate members attached to said tubular members in heat conducting relationship along substantially the entire length thereof, said plate members being arranged relative to each other to form flow channels of alternately converging and diverging cross-sectional area between alternate adjacent pairs of plate members, the minimum cross-sectional flow area of said flow channels being immediately adjacent one of said tubular members, a source of heating fluid, said heating fluid being at a temperature substantially higher than the temperature of the steam in said steam space, means for passing said heating fluid from said source through said tubular members, and batfle means for directing the flow of said steam upwardly through said flow channels whereby said steam is heated slightly above its saturation temperature and contaminant-bearing moisture particles entrained in said steam are collected on said plate members, said moisture being evaporated wh-ile said contaminants remain deposited on said plate members.

4. In a steam generator, an elongated cylindrical steam and water drum of the type wherein a body of liquid having a water level is normally maintained, said water level defining the boundary between a lower water space and an upper steam space, means forming a steam outlet communicating with said steam space, means for discharging a steam-water mixture into said drum below said water level, a primary separating device for separating the major portion of the water from the incoming steam-water mixture, a thermal steam scrubber disposed in said steam space between said primary separating device and said steam outlet including a bank of spaced tubular members extending substantially parallel to the longitudinal axis of the drum, an inlet header and an outlet header disposed within said steam space and respectively connected to said tubular members at their opposite ends, and a plur-ality of closely spaced corrugated thin plate members connected to said tubular members in heat conducting relationship along substantially the entire length thereof, said plate members being arranged relative to each other to form undulating flow channels therebetween, a source of superheated steam at a temperature substantially higher than the temperature of the steam in said steam space, means for passing said superheated steam from said source through said tubular members, baflie means for directing the flow of said steam upwardly through said flow channels, whereby said steam is heated slightly above its saturation temperature and contaminant-bearing moisture particles entrained in said steam are collected on said plate members by virtue of directional changes of steam flow through said flow channels, said moisture being evaporated while said contaminants remain deposited on said plate members, and means disposed above said scrubber for intermittently back-washing said scrubber with Water.

5. Steam purification apparatus comprising a vessel having an inlet and an outlet for flow of steam through said vessel, said inlet being connected for the flow of moisture laden steam into said vessel, a plurality of closely spaced thin plate members disposed in said vessel, said plate members being arranged relative to each other to form undulating flow channels therebetween, means constraining the moisture laden steam flowing into said inlet to flow through said flow channels, and means for heating said plate members to a temperature s bstantially higher than the temperature of the steam entering said vessel, whereby moisture deposited on said plate members in the course of passage through said flow channels is evaporated by heat transferred from said plate members.

6. In a steam generator, an elongated cylindrical steam and water drum of the type wherein a body of liquid having a water level is maintained, said water level defining the boundary between a lower Water space and an upper steam space, means for discharging moisture laden steam into said body of liquid, means forming a steam outlet communicating with the upper portion of said steam space, a plurality of closely spaced thin plate members disposed in said vessel, said plate members being arranged relative to each other to form undulating flow channels therethrough, means constraining the moisture laden steam to flow through said flow channels, means for heating said plate members to a temperature substantially higher than the temperature of the steam entering said vessel, whereby moisture deposited on said plate members in the course of passage through said flow channels is evaporated by heat transferred from said plate members, said means for heating including tubular members connected with said plate members in heat conducting relationship, a desuperheater tube bundle disposed in said water space, a source of superheated steam, means connecting said tubular members and said tube bundle with said source to effect the flow of superheated steam in parallel through said tubular members and said tube bundle, means for regulating the flow of superheated steam through said tubular members to effect a predetermined slight amount of superheat in the steam exiting said drum via said outlet, and means for regulating the total flow of superheated steam in response to a demand for a predetermined quantity of desuperheated steam.

References Cited UNITED STATES PATENTS MOdine 165-151 Kerr 122491 Fletcher 122-491 Wood 122459 Gillette et a1. 122491 1 0 2,387,239 10/ 1945 Bonvilli'an et a1 122-491 3,081,068 3/1963 Milleron 55269 X FOREIGN PATENTS 5 794,450 5/ 195 8 Great Britain. 931,235 7/1963 Great Britain.

SAMIH N. ZAHARNA, Primary Examiner. REUBEN FRIEDMAN, Examiner. 10 R. W. BURKS, Assistant Examiner. 

1. IN A STEAM GENERATOR, A STEAM AND WATER DRUM OF THE TYPE WHEREIN A BODY OF LIQUID HAVING A WATER LEVEL IS NORMALLY MAINTAINED, SAID WATER LEVEL DEFINING THE BOUNDARY BETWEEN A LOWER WATER SPACE AND AN UPPER STEAM SPACE, MEANS FOR DISCHARGING MOISTURE LADEN STEAM INTO SAID BODY OF LIQUID, MEANS FORMING A STEAM OUTLET COMMUNICATING WITH SAID STEAM SPACE, A BANK OF SPACED HEATING ELEMENTS DISPOSED IN SAID STEAM SPACE BETWEEN SAID WATER LEVEL AND SAID STEAM OUTLET, A PLURALITY OF CLOSELY SPACED THIN PLATE MEMBERS ATTACHED TO SAID HEATING ELEMENTS IN HEAT CONDUCTING RELATIONSHIP ALONG SUBSTANTIALLY THE ENTIRE LENGTH THEREOF, SAID PLATE MEMBERS BEING ARRANGED RELATIVE TO EACH OTHER TO FORM UNDULATING FLOW CHANNELS THEREBETWEEN, MEANS FOR HEATING SAID PLATE MEMBERS TO A TEMPERATURE SUBSTANTIALLY HIGHER THAN THE TEMPERATURE OF THE STEAM IN SAID STEAM SPACE, AND BAFFLE MEANS FOR DIRECTING THE FLOW OF SAID STEAM UPWARDLY THROUGH SAID FLOW CHANNELS BETWEEN SAID PLATE MEMBERS, 